The recruitment of cells to the site of inflammation is a key step in a wide range of inflammatory diseases (1-4). This event is mediated by the complex interactions of cellular adhesion molecules (2-8). The leukocyte integrins, of which lymphocyte function associated antigen (LFA- 1;CD11a/CD18) is a member, play a critical role in the recruitment of cells to the site of inflammation. (5,9, 10). In order to mediate these cell- cell adhesive interactions, LFA-1 requires activation and this can be achieved through the stimulation of a number of known signal transduction pathways or by antibodies (11-13). Once activated, lFA-1 exhibits enhanced affinity for its ligand (14). Although a significant understanding of the signals that activate LFA-1 has been achieved, little is understood about the exact mechanisms that result in a change in affinity of lFA-1 for its ligands as well as those signals that are triggered by this receptor ligand interaction. We hypothesize that the leukocyte integrin LFA-1 undergoes a conformational change that results in enhanced binding to its ligand subsequent of intracellular signaling pathways. This event requires specific structural features of the LFA-1 molecular and is associated with the initiation of an intracellular signaling cascade. Our specific objectives are to: 1) Determine the structural features of LFA-1 that are required to mediate the change in affinity of lFA-1 for its ligands. Although the role of the beta subunit in avidity regulation has been well defined, that of the alpha subunit has been hampered by the lack of a suitable cell line that does not express LFA-1 but possesses intact signaling machinery(15). We will utilize a cell line that we have isolated that does not express LFA-1 but has preserved intracellular signaling pathways. We will not only define the minimal structural features of the alpha subunit required for activation of lFA-1 but will also define the specificity of the interaction between alpha and beta subunits. 2) Define signal transduction pathways that are modulated upon the interaction of lFA-1 with its ligand, ICAM-1. This aim will analyze the signals that are generated from the interaction of LFA-1 with its ligand ICAM-1. We will focus our attention to events that are mediated by protein phosphorylation and will utilize an antibody that we have isolated, CBR LFA-1 with ICAM-1 and to define the structural features of lFA-1 that are required for this signaling. Understanding the molecular steps involved with leukocyte integrin activation is critical to a wide array of inflammatory diseases. By defining the molecular details of the recruitment of cells to the site of inflammation we will then be able to, as a long term goal, target steps for therapeutic intervention.